1. Field of the Invention
The present invention relates to an image processing apparatus and, more particularly, to an image processing apparatus having a function of storing an image containing both pixel information of line images such as characters, graphs or the like (referred to as "text", hereinafter) and pixel information of halftone images having gradation such as the image of a photograph.
2. Related Background Art
In general, storage of a text requires a high resolution for ensuring smoothness and continuity of oblique lines and so forth. On the other hand, storage of an image requires a high level of gradation in order to avert any deterioration of the image quality due to false profile. Conventionally, therefore, the memory device of image processing apparatus of the kind described has been constructed to have enough pixels to ensure high smoothness and continuity of oblique lines and enough gradations to prevent degradation of image quality due to false contours as shown in FIG. 4.
According to the conventional memory device described above, enormous numbers of pixels and gradations are necessary for obtaining high qualities of the text and image. This requires a correspondingly large storage capacity of the memory device, resulting in an impractically large scale and cost of the hardware. For instance, when 256 gradation levels are used, 8 bits per pixel are required for expressing gradation. That is, the memory capacity required is 8 times as large that required for storing the text alone which uses one bit for each pixel.
Thus, the conventional memory device is disadvantageous in that a large scale and huge cost are required for the hardware due to employment of great numbers of pixels and gradations, in order to obtain high qualities of the text and image. This problem is serious particularly in the case of storage of full-color images because the required memory capacity is triple, due to the necessity for three planes, for three primary colors, red, green and blue. For instance, when the gradations of each of the R, G and B planes is 256, 24 bits per pixel are necessary for the full-color display, which is 24 times as large that required in the case of storage of monochromatic text, which requires only one bit per pixel.
Under these circumstances, the assignee of the present invention has proposed, in Japanese Patent Application Laid-open No. 62-92070, an apparatus in which resolution is increased for text while gradation is enhanced for image so as to enable both text data to and image data co-exist in the same memory.
Proposed also are techniques in which a representative color in a block is extracted and coded to enable compression of the data in the block, as in U.S. patent application Ser. No. 185,024 filed on Apr. 21, 1988 and U.S. patent application Ser. No. 362,014 filed on Jun. 6, 1988.
These proposals, however, cannot solve the problems of the known art satisfactorily, and there still exists demand for further improvement.